To improve readability and clarity of this application, acronyms are used. A listed of acronyms is provided below:
PON Passive Optical Network
VoIP Voice over Internet Protocol
HDTV High Definition Television
OLT Optical Line Terminal
ONU Optical Network Unit
ODN Optical Distribution Network
TDM Time Division Multiplexing
TDMA Time Division Multiple Access
ID Identification
SOD Start of Data
BD Burst Delimiter
HFP High Frequency Pattern
AGC Automatic Gain Control
CDR Clock and Data Recovery
FSC False Synchronization Candidates
HD Hamming Distance
HDC Hamming Distance Calculation
SDM Synchronization Decision Module
MSB Most Significant Bit
LSB Least Significant Bit
PON is one of the most promising access network technologies. This type of network provides many benefits, including low maintenance cost, high bandwidth, low implementation cost, etc. PON can be an ideal platform for multi-play applications such as VoIP, data transmission, HDTV, etc.
Typically, PON is implemented as a point-to-multipoint medium based on a tree topology including an Optical Line Terminal (OLT), some Optical Network Units (ONUs) and an Optical Distribution Network (ODN) with splitters/couplers. One of the most attractive features of a PON is that the PON does not need any active component in the ODN.
Usually, PON system employs a point-to-multipoint access protocol so that all subscribed ONUs can share an OLT over an optical fiber. For example, the Time Division Multiplexing (TDM) broadcast for downstream transmission and Time Division Multiple Access (TDMA) for upstream transmission is widely used in current PON systems.
As merely an example, FIG. 1 illustrates a downstream transmission process in a PON system. The OLT broadcasts signals to all subscribed ONUs in the downstream transmission, the destination ONU will extract its belonging packets according to the destination identification (ID) of a packet and discard all other packets as in FIG. 1. For example, ONU-1 extracts the packet with its destination ID and sends to its corresponding end user; ONU-2 sends packet-2 to its end user, and so on.
As merely an example, FIG. 2 illustrates an upstream transmission process in a PON system. The ONU transmits its signal in upstream channel of a PON system in a burst mode, which is different from the conventional point to point continuous mode transmission. ONU will first set up a communication link with the OLT, thereafter OLT will allocate different time slots to different ONUs in a TDMA fashion so that their signals will not overlap with each other when they reach the coupler in the ODN. As shown in FIG. 2, the ONU-1 only transmit its signal in its time slot (i.e., No. 1) and ONU-2 transmits its signal in its time slot (i.e., No. 2), and so on.
FIG. 3 is a simplified diagram illustrating a frame structure for upstream data. A high frequency pattern (HFP) “0x 55 55 . . . ” (0x means hexadecimal numbers and its binary form is 01010101 01010101 . . . ) is a special preamble sequence used by the OLT for Automatic Gain Control (AGC) and Clock and Data Recovery (CDR). The HFP is followed by a 66 bits long Start of Data <SOD> delimiter, or it is called Burst Delimiter (BD). The SOD is used to delineate the boundary of the data frame. The length of SOD delimiter is compatible with the 66-bit data frame structure. The section labeled FEC protected “ . . . ” could be one or several IDLE blocks that are used by OLT for de-scrambler re-synchronization and codeword delineation. Data are appended after this (these) IDLE block(s).
The SOD is useful in data synchronization. FIG. 4 is a simplified diagram illustrating the correlation between SOD and False Synchronization Candidates (FSC). After the OLT detects incoming signal and synchronizes the signal with its clock reference, the OLT will send the received signal to the Boundary Detector. A SOD Correlator is embedded within the Boundary Detector module to test the correlation between the SOD delimiter and the received signal. The SOD Correlator will calculate the Hamming distance (HD) between the SOD delimiter and the received 66 bits data to determine the validation of burst synchronization, which is the same as delineate the boundary of the data frame. A false locking synchronization will output a truncated data frame to its higher layer, this may degrade the system performance. Therefore, the employed SOD delimiter should provide a false locking probability as low as possible. The SOD delimiter should be designed to minimize the correlation between the SOD delimiter and the FSC, in other words, maximize the HD between the SOD delimiter and the FSC.
As can be seen from the above description that, various conventional techniques are available for data synchronization in optical networks. Unfortunately, these techniques are often inadequate for various reasons.
Therefore, improved system and method for data synchronization are desired.